MEMS sensors, such as MEMS gyroscopes and accelerometers, are known in the art. In the past few decades, such sensors have drawn great interest. MEMS technology is attractive because, among other reasons, it enables efficient packaging, minimizes sensor area, and significantly reduces power consumption. Further, more specifically, MEMS sensors can be easily integrated with driving and sensing electronics (CMOS-compatible), such that everything can be packaged on the same chip.
Prior art MEMS sensors typically operate in the rate grade. In other words, generally speaking, such MEMS sensors have a rate resolution greater than 0.1°/hr1/2, and require 100 μg for the resolution of detection. Rate grade sensors are useful in certain applications, such as in airbag deployment systems, vehicle stabilization systems, and navigation systems in the automotive industry. But in applications where greater sensor sensitivity is required, rate grade sensors may not be suitable. For example, in applications in the space industry (such as Picosatellites and planetary landers), inertial grade sensors should be used. Inertial grade sensors generally have a rate resolution less than 0.001°/hr1/2, and may require fewer than 4 μg for the resolution of detection.
Prior art MEMS sensors may typically operate in the rate grade due to the configuration of suspensions in the sensor. Typically, such MEMS sensor use in-plane (or horizontal) suspensions (which may be attributable, at least in part, to the fact that such configuration makes it easier and more cost-effective to fabricate such MEMS sensors). The use of in-plane suspensions, however, makes it difficult to obtain inertial grade operation. This may be due to a number of reasons. For example, with such configuration, the suspensions and the proof mass are geometrically coupled to one another. In other words, the dimensions of the suspensions cannot be modified without affecting the geometry of the proof mass. Such configuration also limits the proof mass area fill factor of the sensor (or, in other words, the ratio between the area occupied by the proof mass and the total area of the sensor). This may in turn limit the potential size of the proof mass. Reducing the size of the proof mass may result in, among other things, a degraded Brownian noise floor, an increase in the minimum detectable angular rate, and a worsening of output signal sensitivity to input angular rate, as well as a decease in signal-to-noise ratio (SNR). Further, in such arrangement, out-of-plane deflection may be suppressed, which may, in certain instances, detrimentally affect performance.
The use of out-of-plane suspensions in MEMS sensors, however, significantly improves sensor performance, enabling MEMS sensors to achieve inertial grade operation. Such configuration may do so for a number of reasons. For example, the configuration decouples the suspensions from the proof mass, allowing the dimensions of the suspensions to be optimized without affecting the space available for the proof mass, and, further, significantly improves the proof mass area fill factor of the sensor, as well as the volume fill factor. Such configuration permits a larger proof mass and reduces the resonance frequency and Brownian noise floor, as well as improves the mechanical quality factor, the output signal sensitivity to input angular rate, and SNR.
For the aforementioned reasons and others, there is a need in the art for MEMS sensors (including MEMS gyroscopes and accelerometers) that are inertial grade and/or that use out-of-plane (or vertical) suspensions, as well a method for fabricating such MEMS sensors.